1. Field of the Invention
The invention relates to a method for deep hardening steel workpieces for specifically high surface pressures wherein the workpiece is initially heated to the austenitizing temperature and is then quenched, and to an apparatus for carrying out this method having a heating device for heating the workpiece to the austenitizing temperature and a quenching path through which the workpiece is moved.
2. Description of the Prior Art
Workpieces which absorb high specific surface pressures in use must frequently be subjected to a so-called deep hardening in which hardening depths of 25 mm and more are achieved. When the hardening depth is this large, difficulties occur with respect to the hardening pattern. The desired hardening pattern is one which is uniform and falls off gradually without any great irregularities. However, heretofore, such a desired hardening pattern could not be achieved in a reliably reproducible manner.
In one known method, the deep hardening is achieved by subjecting the workpiece to a deep austenitizing and then quenching the workpiece to produce a martensitic structure. The workpiece is then tempered again using the residual heat still present in the workpiece. However, this method is not satisfactory.
Experiments have shown that a structure of a fine particulate sorbite offers better resistance to wear. However, in order to obtain this structure with as uniform as possible a hardening pattern was not heretofore possible in a reliably reproducible manner.
Experiments were carried out to obtain this structure by means of quenching with compressed air whose amount and pressure is controllable. This makes it possible to obtain a certain critical cooling rate through which high strengths can be obtained due to the formation of the desired sorbite structure. However, it was not possible to obtain a uniform hardening pattern with this method because the control of the compressed air was too slow or sluggish. In addition, the manipulation of the required compressed air is accompanied with such great noise that an industrial application is only possible with expensive protective measures.
Certain improvements in this connection are disclosed in German Offenlegungsschrift No. 26 27 791 wherein a cooling agent is admixed with the quenching medium. This made it possible to significantly reduce the amount of compressed air required and, thus, the annoyance caused by the noise. Moreover, the quenching rate can be controlled with greater sensitivity and speed.
Although it is possible to obtain high strengths by this method, its use has been limited thus far because a uniform hardening pattern could not be reproducibly achieved. Irregularities in the hardening pattern occurred time and again which may lead to crumbling and/or a loss in strength.